In a series of mutation experiments, using both X-rays and EMS, an extensive cytogenetic analysis will be made of both mutant and nonmutant treated chromosomes. The contribution of chromosomal rearrangement to lethality, sterility, and visible mutation will be assessed. The distribution of randomly recovered lethal mutations along the X chromosome will be determined by tests with several different short duplications, extending over more than a third of the X chromosome, that can be used to "cover" induced lethal mutants. Particular regions that appear to be free of induced lethal effects, or to mutate at an especially low rate, will be explored in the hope of finding evidence for the existence of persisting duplications in the normal genome. Futher, the difference between mutant and nonmutant rearrangement breakpoints will be investigated. Also, the detailed relationships of specific genes to particular salivary chromosome bands will be studied, as will the mutability of genes associated with large bands in comparison with that of genes associated with thin, delicate bands. the distribution of EMS-induced mutations will be compared with that of X-ray-induced mutations. In all experiments, the mutational consequences of treating germ cells at different stages of maturity will be compared (for both chromosomal and nonchromosomal mutants) by testing, separately, each successive brood of progeny produced after treatment. The overall objective is to gain a better understanding of the genetic organization of eukaryotic chromosomes.